Method and apparatus for manufacturing Si-containing cold rolled steel sheets

Abstract

This invention provides methods and apparatuses which can manufacture Si-containing cold rolled steel sheets exhibiting excellent chemical conversion properties even if a chemical conversion treatment solution is used at a lower temperature while minimally suppressing the generation of sludge as well as reducing running costs. The method includes steps of cold rolling a steel containing 0.5 to 3.0 mass % Si, continuously annealing the cold rolled steel sheet, pickling the surface of the continuously annealed cold rolled steel sheet, and repickling the surface of the pickled steel sheet with a non-oxidative acid. The repickling is performed such that a repickling solution is sampled continuously or periodically, an acid concentration in the sampled solution is measured, and the acid concentration in the repickling solution is regularly controlled within a prescribed concentration range.

The invention claimed is:  
     
       1. A method for manufacturing Si containing cold rolled steel sheets with excellent chemical conversion properties, comprising steps of:
 cold rolling a steel sheet containing 0.5 to 3.0 mass % Si, continuously annealing the cold rolled steel sheet, pickling a surface of the continuously annealed cold rolled steel sheet, and repickling the surface of the pickled steel sheet with a non-oxidative acid, 
 wherein the repickling is performed such that: 
 a repickling solution is sampled continuously or periodically; 
 an acid concentration in the sampled solution is measured; and 
 the acid concentration in the repickling solution is regularly controlled within a prescribed concentration range. 
 
     
     
       2. The method for manufacturing Si containing cold rolled steel sheets according to  claim 1 , wherein:
 the acid concentration in the repickling solution is measured by one of a near infrared spectroscopic analysis method, a glass electrode method, and an electromagnetic induction method. 
 
     
     
       3. The method for manufacturing Si-containing cold rolled steel sheets according to  claim 1 , wherein:
 the acid concentration in the sampled repickling solution is measured after the sampled solution is passed through a filter. 
 
     
     
       4. The method for manufacturing Si containing cold rolled steel sheets according to  claim 3 , wherein:
 a pore diameter of the filter is not less than 20 μm and not more than 30 μm. 
 
     
     
       5. The method for manufacturing Si containing cold rolled steel sheets according to  claim 1 , wherein:
 the acid used in the repickling is one of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid, and a mixture of two or more thereof. 
 
     
     
       6. The method for manufacturing Si containing cold rolled steel sheets according to  claim 1 , wherein:
 the acid used in the repickling is one of hydrochloric acid with a concentration of 0.1 to 50 g/L, sulfuric acid with a concentration of 0.1 to 150 g/L, and a mixture of hydrochloric acid and sulfuric acid with respective concentrations of 0.1 to 20 g/L and 0.1 to 60 g/L. 
 
     
     
       7. The method for manufacturing Si containing cold rolled steel sheets according to  claim 1 , wherein:
 the repickling is performed at a temperature of the repickling solution of 20 to 70° C. for a repickling time of 1 to 30 seconds. 
 
     
     
       8. The method for manufacturing Si containing cold rolled steel sheets according to  claim 1 , wherein:
 the pickling is performed using a pickling solution that is one of a mixture of nitric acid and hydrochloric acid with respective concentrations of above 50 g/L and not more than 200 g/L and above 1 g/L and not more than 200 g/L, and a mixture of nitric acid and hydrofluoric acid with respective concentrations of above 50 g/L and not more than 200 g/L and above 1 g/L and not more than 200 g/L. 
 
     
     
       9. The method for manufacturing Si containing cold rolled steel sheets according to  claim 1 , wherein:
 the acid concentration in the pickling solution is measured with a lower accuracy and/or at a longer interval compared with an accuracy and an interval for measuring the acid concentration in the repickling solution. 
 
     
     
       10. An apparatus for manufacturing Si containing cold rolled steel sheets with excellent chemical conversion properties, comprising:
 a pickling tank for pickling a surface of a steel sheet that is cold rolled and continuously annealed, 
 a repickling tank for repickling the surface of the pickled steel sheet with a non-oxidative acid, 
 a measurement unit that continuously or periodically samples a repickling solution and measures an acid concentration in the sampled solution, and 
 a unit for regularly controlling the acid concentration in the repickling tank within a prescribed concentration range based on the measured results. 
 
     
     
       11. The apparatus for manufacturing Si containing cold rolled steel sheets according to  claim 10 , wherein:
 the measurement unit utilizes one of a near infrared spectroscopic analysis method, a glass electrode method, and an electromagnetic induction method. 
 
     
     
       12. The apparatus for manufacturing Si-containing cold rolled steel sheets according to  claim 10 , wherein:
 the measurement unit includes a filter for removing suspended solids before the acid concentration in the sampled solution is measured. 
 
     
     
       13. The apparatus for manufacturing Si-containing cold rolled steel sheets according to  claim 12 , wherein:
 a pore diameter of the filter is not less than 20 μm and not more than 30 μm. 
 
     
     
       14. The apparatus for manufacturing Si-containing cold rolled steel sheets according to  claim 10 , wherein:
 the acid used in the repickling is one of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid, and a mixture of two or more thereof. 
 
     
     
       15. The apparatus for manufacturing Si-containing cold rolled steel sheets according to  claim 10 , wherein:
 the acid used in the repickling is one of hydrochloric acid with a concentration of 0.1 to 50 g/L, sulfuric acid with a concentration of 0.1 to 150 g/L, and a mixture of hydrochloric acid and sulfuric acid with respective concentrations of 0.1 to 20 g/L and 0.1 to 60 g/L. 
 
     
     
       16. The apparatus for manufacturing Si-containing cold rolled steel sheets according to  claim 10 , wherein:
 the repickling is performed at a temperature of the repickling solution of 20 to 70° C. for a repickling time of 1 to 30 seconds. 
 
     
     
       17. The apparatus for manufacturing Si-containing cold rolled steel sheets according to  claim 10 , wherein:
 the pickling is performed using a pickling solution that is one of a mixture of nitric acid and hydrochloric acid with respective concentrations of above 50 g/L and not more than 200 g/L and above 1 g/L and not more than 200 g/L, and a mixture of nitric acid and hydrofluoric acid with respective concentrations of above 50 g/L and not more than 200 g/L and above 1 g/L and not more than 200 g/L. 
 
     
     
       18. The apparatus for manufacturing Si-containing cold rolled steel sheets according to  claim 10 , wherein:
 an acid concentration in the pickling solution is measured with a lower accuracy and/or at a longer interval compared with an accuracy and an interval for measuring the acid concentration in the repickling solution.